1. Field of the Invention
The present invention relates to a method for forming a scale preventive coating on inner walls of a polymerization vessel, and in particular relates to a method for forming a scale preventive agent coating which is applied to a polymerization vessel for use in polymerization of a monomer having an ethylenically unsaturated double bond.
2. Description of the Prior Art
In the prior art, methods for polymerization of a monomer having an ethylenically unsaturated double bond have been known, such as suspension, emulsion, solution, gas phase and bulk polymerization processes and the like. In any of these polymerization processes, polymer scale is liable to be deposited on the areas with which a monomer comes into contact, such as inner walls, stirring equipment and so on of a polymerization vessel.
The deposition of the polymer scale results in disadvantages that the yield of the polymer and cooling capacity of the polymerization vessel are lowered, and that the polymer scale may peel off and mix into a polymeric product, thereby impairing the quality of formed products obtained by processing the polymeric product. In addition, removal of deposited polymer scale is very laborious and time-consuming. Further, the polymer scale contains unreacted monomers, and thus an operation of scale removal involves a risk of physical disorders.
Heretofore, a method for preventing polymer scale deposition on inner wall surfaces of a polymerization vessel has been known, that is a method in which a coating of a polymer scale deposition preventive agent is formed on inner wall surfaces and so on of a polymerization vessel.
As such a method, for example, Japanese Patent Publication (KOKOKU) No. 1-31522 discloses a method in which a liquid scale preventive agent is applied by spraying to inner walls of a polymerization vessel by using an application apparatus including a two-fluid (liquid-gas) spray nozzle arranged inside the polymerization vessel.
Further, Japanese Patent Publication (KOKOKU) No. 1-38544 proposes a method in which vinyl chloride gas is used as a carrier, and a coating substance (scale preventive agent) is coated on surfaces of a reaction vessel in a mist state by using an appropriate spray nozzle arranged in the reaction vessel.
However, when polymerization is performed in an industrial scale, a large polymerization vessel having large capacity is generally used. Further, from viewpoints of influence of monomer gas to human body and loss of monomer gas and so on, it is desired to continuously perform polymerization in a closed polymerization system without opening a polymerization vessel.
However, in the case of a method in which a scale preventive agent is applied by means of a spray procedure as in the prior arts described above, it is difficult to uniformly and evenly form a scale preventive coating on inner wall surfaces of a large polymerization vessel having a volume, for example, not less than 80 m.sup.3. Namely, a spray nozzle for spraying a scale preventive agent is generally arranged at an upper portion in a polymerization vessel, however, a distance between the nozzle and the bottom of the polymerization vessel becomes longer as the polymerization vessel becomes large, and thus it becomes difficult to transport an enough amount of droplets of the scale preventive agent coating liquid to the bottom of the polymerization vessel. As a result, it becomes difficult to uniformly form a scale preventive coating on inner walls of the polymerization vessel.
The tendency as described above becomes more conspicuous in a closed polymerization system. For example, when spraying is performed by using a spray nozzle in a closed polymerization vessel, the gas in the vessel is subjected to retention at a lower portion of the polymerization vessel, and a flow of gas to reach the lower portion of the polymerization vessel becomes difficult to occur. Thus it becomes more difficult to apply a sufficient amount of a scale preventive agent to lower portions of inner wall surfaces of a polymerization vessel, a lower portion of a stirring shaft, an opening valve and peripheral portions around a valve seat, and it is impossible to effectively prevent occurrence of scale deposition on these portions.
Therefore, the methods of the prior art described above are not suitable for industrial production. Especially, the scale deposition on the opening valve and peripheral portions around the valve seat is a serious problem, wherein opening and closing operations of the opening valve even become impossible during a procedure of repeated polymerization. In such a case, means of lengthening the spray time, increasing the spray amount and so on may be thought of in order to form a sufficient scale preventive coating on lower portions of a polymerization vessel, however, any of them has large disadvantages in time and economy, and cannot provide a fundamental solution.